feat: allow specifying a custom font for the card text

[shuuji3]

resolve #108

Hello, this PR implements the custom font setting and the default "Roboto" font. I encountered the same issue as #108 when I tried to use this for the Japanese translation of psycopg3 documentation. This should be helpful for any Sphinx project written in non-Latin languages.

I validated this change on the Debian container with Noto CJK font installed by apt install fonts-noto-cjk. Here are examples:

Before

After

I think this won't change the default Roboto Flex fallback behavior, but let me know if this needs any adjustment. Thanks.

[zhudotexe]

Awesome! @Daltz333 is there anything blocking merging this PR? We'd love to be able to use the social image card generation.

[shuuji3]

Thanks! 😃